Driver assist camera systems are increasingly being provided in modern vehicles to provide the driver with assistance, for example in driving or in parking the vehicle. In particular, driver assist systems are able to provide a driver with a view of the position of the vehicle, relative to other vehicles or to features of the road such as the curb, to help the driver in maneuvering or parking the vehicle. In some driver assist camera systems, graphical data is displayed to the user, for example to indicate the projected path of the vehicle on the ground when parking or overtaking.
Driver assist camera systems typically include one or more cameras, mounted on the vehicle, that capture images of the surroundings of the vehicle. The images captured by the camera are calibrated so that the images accurately represent the surroundings of the vehicle, and so that they can be combined with other images and/or with graphical data for display. Driver assist camera systems typically perform a geometrical projection of a camera image onto a plane representing the ground on which the vehicle is located, and may perform an overlay of additional graphical data, such as graphical data illustrating the projected path of the vehicle on the ground, on the image data.
The height of the camera relative to the ground when the image was taken and the height of the camera relative to the projection plane onto which the image is to be projected are used to perform an accurate geometrical projection of an image on to a plane. Inaccuracy in this height information makes the alignment of composite pictures made by combining images from more than one camera more difficult, because the edges or overlap between the original images will not correspond. In addition, the positioning of graphical information overlaid on a camera image or on a composite camera image can be inaccurate.
In some driver assist camera systems, the height of the camera above the ground is measured in a calibration stage during manufacture. Although this method is simple to carry out, the height measurement becomes inaccurate with load, ride height changes and other variations.
In other driver assist camera systems, a camera height measurement for use in calibrating images may be updated in service from the images taken by the camera. One such system is disclosed in WO2009/142921, which is incorporated by reference, which relates to a system and a method for calibrating a camera on a vehicle while the vehicle is being driven. Images from a camera mounted on the vehicle are analysed to determine a ground plane, and the height of the camera above the determined ground plane can be found.
Although systems such as this can produce good results, the processing of the images to determine the height of the camera may take some time, and the height of the camera tends to be updated infrequently. In addition, techniques to analyse the position of a ground plane from images taken by a camera may work poorly on some surfaces. Furthermore, rotations or deflections of the camera caused by load or handling variations are not always considered. These factors mean that the calibration of images from the camera may be inaccurate for periods of time.
The present invention seeks to alleviate or ameliorate at least some of the disadvantages of the prior art, and to provide a method of calibrating images.